Articular cartilage is a specialized connective tissue that bears load and reduces friction across moving joints. It is composed of an extracellular matrix that contains no nerves or blood vessels and relatively few cells (5% volume). Damage can arise due to disease or trauma and is common, especially in the aging population. Cartilage can decrease in strength with age. When damaged, articular cartilage either does not heal or at best heals only very slowly. Instead of healing, damaged cartilage often degenerates further, leading to pain and loss of function. Due to the prevalence of osteoarthritis (OA) and damage to articular cartilage, coupled with this poor intrinsic healing response, there is a great demand for clinical intervention.
Treatment of damaged cartilage in living animals presents difficult challenges. Adult cartilage is difficult to repair. Joint repair is conventionally done by replacing the entire joint or joint surfaces without trying to repair cartilage, usually in the form of a highly-invasive non-biological prosthetic (such as total joint arthroplasty). However, metal/plastic orthopedic implants have a limited lifespan (e.g., about 20 years) and are ideally reserved for use in older patients. The onset of arthritis, however, can begin as early as the age of 40 with much younger patients suffering from the disease as a result of trauma. Thus, some patients face the prospect that an artificial joint implant may wear out and need to be replaced.
One common biological alternative to arthroplasty entails the transplantation of healthy osteochondral autografts (cartilage along with some of the underlying bone) from a non-load bearing region. Osteochondral implants are designed to be press-fit into pre-drilled cavities in the damaged joint, replacing the host cartilage above while anchoring to the bone below. Osteochondral grafts are better anchored than chondral-only grafts and are less likely to be displaced by shearing forces within the joint. While these autologous grafting procedures are promising, they are limited both by the amount of tissue available and donor-site morbidity associated with its harvest. The use of donor cartilage from tissue banks (allografts) or from animal origin (xenografts) addresses these limitations, but introduces the possibility of disease transmission.
Tissue engineering strategies, if successful, could alleviate these problems by creating replacement tissues of the proper size and shape without concurrent damage to other regions of the patient's body. There is a great variety of tissue engineering approaches to form osteochondral constructs. For example, techniques for repairing cartilage have been proposed using scaffolds implanted with progenitor cells such as chondrocytes, stromal cells, stem cells, and such. However, clinical outcomes with biologic replacement materials have not been satisfactory, particularly because of mechanical issues, morphology and durability of biologics-based replacements.